1. Field of the Invention
The present invention relates to a portable magnet power supply for a superconducting magnet comprising apparatus for the storage of energy released from a superconducting magnet, useful in the controlled management of rundown energy released from superconducting magnets. In particular, it relates to apparatus for the storage of such energy within the portable magnet power supply for later dissipation.
2. Description of the Prior Art
Superconducting magnets are well known, and find application in Magnetic Resonance Imaging (MRI) systems, particle accelerators, Nuclear Magnetic Resonance (NMR) spectroscopy, energy storage and other applications. In use, an electric current flows essentially losslessly in a closed superconducting circuit. It is required to remove the current from superconducting magnets at certain times, for instance to enable service operations. Such intentional removal of current is known as ramping down.
FIG. 1 shows a schematic approximate equivalent circuit for a superconducting magnet. Within a cryogenic enclosure 10, at least one coil 12 of superconducting wire is provided, with accessible electrical connections 14, 16. In parallel with the coil(s) 12 is a superconducting switch 18. The superconducting switch comprises a length of superconductive wire, typically sheathed in a resistive metal outer 18b. A protection diode, or combination of diodes, 18c is typically connected across the switch. A small heater 18d is provided in thermal contact with the superconducting wire 18a. When required, an electric current is passed through the heater 18d, which heats the superconducting wire sufficiently to cause it to quench, becoming resistive. Electric current through the switch 18 must then pass through the resistive sheathing or through the quenched superconductor.
Conventionally, ramping down proceeds by connecting a portable magnet power supply across terminals of the magnet, and opening a superconducting switch within the magnet to cause the magnet current to flow through the magnet power supply. Typically, within the magnet power supply is provided a very high power diode arrangement, which causes a voltage drop within the magnet current path. This voltage drop, in combination with the magnet current flowing through it, leads to dissipation of energy as heat. This heat is carried from the diode to a heat sink provided within the magnet power supply for the purpose. As heat is dissipated in the diode, the heat sink is warmed, and dissipates heat to ambient, mainly by convection, but also by radiation and conduction. The heat sink must be large and massive, in order to dissipate heat at the rate that it is produced by the diode. This conventional arrangement requires a large, heavy-diode-and-heat-sink arrangement to ensure that is does not overheat when dissipating the energy stored in a superconducting magnet. In a typical current example, a 1.5 T superconducting magnet may store 4 MJ of energy, with the magnet power supply being designed to dissipate this energy in about 30 minutes. This represents an average dissipation power of 2.2 kW, but a peak dissipation power much higher. The energy of a 3 T magnet may take three times as long, using the same run-down load. In some known magnet power supplies, the diode is replaced by a resistor.
FIG. 2 schematically illustrates an approximate equivalent circuit of a conventional portable magnet power supply 20. The power supply 20 has externally accessible connections 24, 26 for connection to the connections 14, 16 of the magnet 10. A power converter 22 receives 3-phase mains power 24 and converts this into a low-voltage, high current DC output. A run-down load 28 is provided, in thermal contact with a large heat sink 30. The heat sink 30 is typically a metal block, and is usually provided with fins 32 and a fan 34 to aid cooling. A mode switch 36 allows a user to switch between a ramping mode, in which the power converter 22 is connected across the magnet 10, and a run down mode, in which the run-down load 28 is connected across the magnet.
Known portable magnet power supplies are large and heavy, typically weighing about 85 kg. Service technicians transport these power supplies around the world to service superconducting-magnet-containing systems such as MRI systems. It is desired to reduce the size and weight of such power supplies. It is important to minimize the size and weight of the magnet power supply, as transportation costs represent a large portion of the costs of a service call.
Arrangements for temperature stabilization using phase-change materials are described in United States patent application 2002/0020174 and Japanese patent application JP60189021. Further discussion of such applications may be found in “Thermal Management Using “Dry” Phase Change Materials”, Proc. Fifteenth IEEE Semiconductor Thermal Measurement and Management Symposium, Mar. 9-11, 1999, San Diego Calif. pp 74-82 IEEE No. 99CH36306.